This invention relates to wind turbines, and in particular to wind turbines having a vertical axis.
For eons, wind power has been a source of energy and has been harnessed in various fashions. A profound distinction can be made between horizontal axis wind turbines (HAWTs) and vertical axis wind turbines (VAWTs).
In modern times, the prevalent concept is the HAWT, most frequently with three airfoil blades. While this type of wind turbine claims the highest efficiency, they are burdened with some substantial disadvantages:                a. They are mono-directional, which means they have to be turned into the wind.        b. Their minimum operational wind speed (cut-in speed) is relatively high and the maximum wind speed (cut-out speed) they can endure is relatively low, allowing for only a relatively narrow window of operation, beyond which they are prone to damage and have to stop operating.        c. Their serviceable components sit high up in the so-called nacelle, on top of a tall pillar, which is inconvenient for service and parts replacements.        d. Although they are considered “fast-runners” by the yardstick of their lift factor, their actual slewing speed is relatively low (typically in the range of 15 to 30 RPM), which necessitates expensive multi-stage gearboxes, which impacts negatively on the overall system efficiency and costs.        e. Many people consider HAWTs to have an ugly appearance and to be an insult to the landscape.        f. By their design, HAWTs do not lend themselves to do-it-yourself construction and actually grow attractive with size.        
Nowadays commercial applications are but all exclusively covered by HAWTs. However, VAWTs avoid most of the above disadvantages, in that:                a. They are omni-directional.        b. The useful cut-in wind speed is lower and the cutout speed is higher, thus making the window of operation wider.        c. Their serviceable components can be concentrated at their bottom end and for easy accessibility.        d. Also considered “low runners” by their low lift factor, they actually slew faster, allowing for smaller-ratio gearboxes, which are less expensive and more efficient.        e. They allow for more flexible, pleasing designs.        f. They allow for operation at higher wind speeds (storms) with lower risk of getting damaged.        g. They lend themselves to simple design and construction.        
The main disadvantage of omni-directional VAWTs is their substantially lower efficiency; the latter defined as the ratio between the latent energy in the wind and the actual power output.